The present invention relates to positioners for multiple pairs of load-handling forks mounted on a lift truck, such multiple pairs including at least an outer pair of forks and an inner pair located between the outer pair. Multiple-pair fork positioners enable a lift truck to selectively carry either a single palletized load or multiple palletized loads in side-by-side relation, depending upon the adjustable positions of the forks.
Some types of multiple-pair fork positioners have required a separate powered actuator for each fork in order to adjust the positions of the forks. However, such a multiplicity of actuators adds unnecessary weight and expense to the lift truck's load-handling assembly. In order to limit the number of powered actuators required, other multiple-pair fork positioners have provided powered actuators only for the outer pair of forks, while the inner pair of forks are selectively moved in unison with the outer pair of forks over a limited range of movement. For example, U.S. Pat. No. 4,533,290 and Austrian Patent No. 342500 disclose positioners which provide limited extension and retraction of the inner forks in unison with the extension and retraction of the outer forks. However, neither discloses a structure capable of fixedly maintaining a constant spacing between the adjacent inner and outer forks in the outer range of extension of the outer forks. Likewise, German published patent application No. DE 3702918, although disclosing stops on the outer fork slides which pull the inner forks outwardly with the outer forks to the full extension of the outer forks, provides only a tension spring to prevent any narrowing of the spacing between the adjacent inner and outer forks. Therefore, obstacles or frictional binding of the inner forks during retraction of the outer forks can cause inadvertent narrowing of such spacing merely by overcoming the spring force.
Any fork positioner which does not fixedly prevent both widening and narrowing of the spacing between the adjacent inner and outer forks, during extension or retraction of the outer forks in their outer range of extension, can cause severe load handling problems. For example, if the forks are supporting a palletized load, a widening of the spacing during extension, caused by an obstacle or frictional binding impeding the extension of the inner fork, may pull the pallet apart and cause the load to fall off of the forks. Conversely, a narrowing of the spacing during retraction may destabilize the support of the load by the forks, likewise causing the load to fall off of the forks. Alternatively, if the forks are unloaded during such extension or retraction, the positioning of adjacent inner and outer forks to engage a pallet may be rendered difficult or impossible if the fork spacing is varied by obstacles or frictional binding impeding the movement of the inner fork, since the spacing of the adjacent forks may then not match the spacing of the pallet fork pockets.
A multiple-pair fork positioner has been previously marketed by Cascade Corporation of Portland, OR which provides a hinged linkage between adjacent inner and outer forks. The hinged linkage, in response to extension of the outer forks, extends to a locked position which, upon locking, bidirectionally maintains a fixed spacing between the inner and outer forks during their subsequent extension and retraction within the outer range of extension of the outer forks. However, such linkage does not provide a detachable connection between the adjacent inner and outer forks and therefore has substantial length which is susceptible to bending or other damage by contact with loads or other obstacles. Moreover, the linkage does not permit any adjustment in the fixed spacing between the adjacent inner and outer forks to accommodate different spacings of pallet fork pockets or different sized loads.